I wasn't intending to poke a little but that was the first thing that jumped into my head. As far as I know, there wasn't anything mounted to those holes from the factory. We did use them to mount fog lamps and other objects such as a spoiler or cb radio external speaker. Fog lamps and a speaker are out since you didn't find any wiring running to them.
I tend to agree that they were put there to help in assembly. Maybe to hang on a parts rack.
